As mentioned earlier, if you’ve got the old metal galvanized pipes, you’re probably experiencing loss of water pressure and occasional chunks of rust coming through your waterlines. The only way to repair it is to replace it.
When installing new pex lines, never worry about removing the old waterlines. If they aren’t in the way, let them lie.
The toughest part of replacing waterlines is working underneath the home in cramped space. But before you start doing the back stroke, be sure you understand how your home’s plumbing system functions. Basically, there’s not much to understand. In fact it can all be summed up in one paragraph.
The ¾” main waterline comes into the house by the water heater and makes a tee. One side of the tee goes into the hot water heater which then feeds all the ½” hot waterlines. The other side of the tee feeds all the ½” cold waterlines. Congratulations! Go pick up your diploma as you just passed Plumbing 101.
Codes
Manufactured homes are built to a HUD code. Homes built to a UBC or any other code have stricter plumbing requirements. In most areas pex waterlines are acceptable but all fittings have to be accessible. In other words, connections cannot be made inside walls or underneath floors as they typically are in a manufactured home.
For pex to meet codes under those conditions, manifolds are used. Think of a manifold as a hot and cold control panel. The hot side of your main line would flow into the hot control panel. The cold side would flow into the cold control panel. A separate waterline would run from the control panel to each fixture in house. For instance, in your bathroom three ½” cold waterlines would run from the cold panel to your bathroom sink, tub and toilet. To accomplish that and meet code, each waterline would need to be one long piece with no hidden fittings connecting additional pieces of waterline. The beauty of this system is that you wouldn’t need to install shut-off valves at each fixture, but rather simply turn off the valve for that line at the manifold. Depending upon your area, your waterlines may need to be inspected by a building inspector. Find out before starting any major work. If your area is like most, building inspectors don’t require mobile homes to meet any code but HUD.
Making a crimp connection 
Step 1 – Using a pipe cutter, make a square cut. Remove any burrs or jagged edges.
Step 2 – Slide the correct-sized crimp ring over the end of the pex waterline.
Step 3 – Insert the fitting into the pipe until it hits the shoulder. The ring should be positioned 1/8 to ¼” from the end of the pipe. If you encounter difficulty with keeping the ring in place until crimped, gently squeeze the ring using a pliers. Do not oversqueeze with the pliers or you may not be able to get the cimping tool over the ring.
Step 4 – With a properly calibrated crimper, squarely center the jaws over the ring and squeeze the handle one time. If crimped more than once, the connection must be cut out and redone.
Step 5 – Remove crimper and check the ring using a GO/NO-GO gauge. A GO/NO-GO gauge will tell you if you’ve made a proper crimp. For each size of pipe the gauge will have two slots. The GO slot will slip over the ring, the NO-GO slot will not. If both slots (or neither slot) slip over the ring, then the connection must be cut out and redone once the crimper has been recalibrated. To recalibrate a crimper, one screw loosens and the other adjusts. Since all brands of crimpers adjust a bit differently, refer to your manual for more details.
Waterlines - Understand and Install | Tips for Installing New Waterlines
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